Background: Physical Vapor Deposition (PVD)
Physical vapor deposition (PVD) processes (often just called thin film processes) are atomistic deposition processes in which material is vaporized from a solid or liquid source in the form of atoms or molecules, transported in the form of a vapor through a vacuum or low pressure gaseous (or plasma) environment to the substrate where it condenses. FIGS. 1A and 1B are general schematics of the PVD process. Typically, PVD processes are used to deposit films with thicknesses in the range of a few nanometers to thousands of nanometers; however, they can also be used to form multilayer coatings, graded composition deposits, very thick deposits, and freestanding structures. The substrates can range in size from very small to very large such as the 10′ by 12′ glass panels used for architectural glass. The substrates can range in shape from flat to complex geometries such as watchbands and tool bits. Typical PVD deposition rates are 10–100 Å (1–10 nanometers) per second.
PVD processes can be used to deposit films of elements and alloys as well as compounds using reactive deposition processes. In reactive deposition processes, compounds are formed by the reaction of depositing material with the ambient gas environment such as nitrogen (e.g. titanium nitride, TiN) or with a co-depositing material (e.g. titanium carbide, TiC). Quasi-reactive deposition is the deposition of films of a compound material from a compound source where loss of the more volatile species or less reactive species during the transport and condensation process is compensated for by having a partial pressure of reactive gas in the deposition environment. A more in-depth review of the PVD process can be found in the Handbook of Physical Vapor Deposition (PVD) Processing—Film Formation, Adhesion, Surface Preparation and Contamination Control, Soc. Of Vacuum Coaters, Albuquerque, N. Mex. (1998) by Donald M. Mattox which is hereby incorporated by reference.
Background: Ion Implantation
Ion implantation is a high technology approach for modifying surface properties of materials. It is similar to a coating process, but it does not involve the addition of a layer on the surface. Ion implantation utilizes highly energetic beams of ions (positively charged atoms) to modify the surface structure and chemistry of materials at low temperature. The process does not adversely affect component dimensions or bulk material properties.
Many surface properties can be improved with ion implantation including hardness and wear resistance, resistance to chemical attack, and reduced friction. The process can be applied to virtually any material, including most metals, ceramics, and polymers. However, the effects of the process are typically material-specific.
The ion implantation process is conducted in a vacuum chamber at very low pressure (10−4–10−5 torr). Large numbers of ions (typically 10−16–10−17 ions/cm2) bombard and penetrate a surface, interacting with the substrate atoms immediately beneath the surface. Typical depth of ion penetration is a fraction of a micron (or a few millionths of an inch). The interactions of the energetic ions with the material modify the surface, providing it with significantly different properties than the remainder of the material. Specific property changes depend on the selected ion beam treatment parameters, for instance the particular ion species, energy, and total number of ions that impact the surface.
Ions are produced via a multi-step process in a system such as that shown schematically in FIG. 2. Ions are initially formed by stripping electrons from source atoms in a plasma. The ions are then extracted and pass through a mass-analyzing magnet, which selects only those ions of a desired species, isotope, and charge state. The beam of ions is then accelerated using a potential gradient column. Typical ion energies are 10–200 keV. A series of electrostatic and magnetic lens elements shapes the resulting ion beam and scans it over an area in an end station containing the parts to be treated. Ion implantation offers numerous advantages for treating component surfaces. A primary benefit is the ability to selectively modify the surface without detrimentally affecting bulk properties, largely because the process is carried out at low substrate temperatures. The process is also extremely controllable and reproducible and can be tailored to modify different surfaces in desired ways. Although it is a line-of-sight process, specialized fixturing may be used to uniformly treat complex geometries.
Background: Ion-Beam Assisted Deposition (IBAD)
Ion-beam assisted deposition (IBAD) utilizes two physical processes in high vacuum: ion implantation and physical vapor deposition (PVD).
In IBAD, metal or oxide targets are placed at the evaporator and used for thin film deposition. The ion beam can be generated from noble gases or from gases like nitrogen or oxygen (with additional chemical influence on the layer deposition leading to changed stoichiometry of nitrides or oxides).
Ion bombardment is the key factor controlling film properties in the IBAD process. As in ion implantation, the ions impart substantial energy to the coating and coating/substrate interface. This achieves the benefits of substrate heating (which generally provides a denser, more uniform film) without significantly heating the substrate material and degrading bulk properties. The ions also interact with coating atoms, driving them into the substrate and producing a graded material interface, which enhances adhesion. These factors combine to allow the deposition of uniform, adherent, low-stress films of virtually any coating material on most substrates, including extremely adherent metal coatings on polymers. Therefore, IBAD allows high quality depositions, where conventional PVD coatings fail. FIG. 3 is a general schematic of the IBAD process.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling. In a conventional drill rig, as seen in FIG. 4, a drill bit 10 is mounted on the end of a drill string 12 (drill pipe plus drill collars), which may be several miles long. At the surface, a rotary drive turns the string, including the bit 10 at the bottom of the hole, while drilling fluid (or “mud”) is pumped through string 12 by very powerful pumps.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called “tripping”: a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one “stand” of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons and the length of the drill string may be tens of thousands of feet, this is not a trivial job. One trip can require tens of hours and is a significant expense in the drilling budget. To resume drilling, the entire process must be reversed. Thus, the bit's durability is very important to minimize round trips for bit replacement during drilling.
Two main types of drill bits are in use; one being the roller cone bit. FIG. 5 shows an example of a complete bit (of the insert type) in which a set of rotary cones 14, each having many teeth or cutting inserts 16, is mounted on rugged bearings on an arm 18. The bit's teeth must crush or cut rock with the necessary forces supplied by the “weight on bit” (WOB) which presses the bit down into the rock and by the torque applied at the rotary drive. While the WOB may in some cases be 100,000 pounds or more, the forces actually seen at the drill bit are not constant: the rock being cut may have harder and softer portions (and may break unevenly), and the drill string itself can oscillate in many different modes. Thus, the drill bit must be able to operate for long periods under high and variable stresses in a remote environment.
As the drill bit rotates, the roller cones roll on the bottom of the hole. The weight-on-bit forces the downward pointing teeth of the rotating cones into the formation being drilled, applying a compressive stress which exceeds the yield stress of the formation and thus inducing fractures. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid.
Background: The Importance of Seals in Drilling
The flow of the mud is one of the most important factors in the operation of the drill bit, serving both to remove the cuttings which are sheared from rock formations by the drill bit and also to cool the drill bit and teeth (as well as other functions). However, the fragments of rock in the mud (which are constantly being released at the cutting face) make the mud a very abrasive fluid.
At least one seal is normally designed into the arm/cone joint to exclude the abrasive cuttings-laden mud from the bearings.
FIG. 6 is a sectional view of a portion of a roller cone bit. Seen in outline is the external surface of the roller cone 10, while the journal 20 with roller bearings 22 and ball bearings 24 are seen as they fit in the cone. The seal 26 and gland 28, which lie within the cone as it rotates around the journal, are seen in a cut-away that shows their cross-section.
The special demands of sealing the bearings of roller cone bits are particularly difficult. The drill bit is operating in an environment where the turbulent flow of drilling fluid, which is loaded with particulates of crushed rock, is being driven by hundreds of pump horsepower. The flow of mud from the drill string may also carry entrained abrasive fines. When the seal fails, the abrasive cuttings-laden mud will very rapidly destroy the bearings. Thus, the seal is a very critical factor in bit lifetime and may indeed be the determining factor.
Background: Prior Attempts at Improving the Wear Resistance of Drill Bit Seals
Prior attempts at improving the wear resistance of drill bit seals include adding an organic material to the surface of the seal. For example, in U.S. Pat. No. 5,456,327 to Denton et al., the surface of the seal was modified with organic materials such as metal disulfides, fluoropolymers, polyethylene polymers, silicone polymers, and urethane polymers. These organic materials improved the wear resistance of the seals by minimizing sticking between the seal surface and adjacent surfaces which minimizes the material loss at the seal surface resulting from stick-slip. Other examples of seals treated with an organic material include the Fluoro-elastomer 742 seal from Chemraz™.
Background: Shortcomings of Organic Coatings
Organic coatings improve wear resistance of the seal at the dynamic surface by reducing the sticking between the seal surface and the adjacent surfaces. However, these coatings fail to provide support for the soft seal surface against a harder surface. The soft surface of the elastomeric seal is still in contact with hard surfaces.
Also, organic coatings do not protect the seal against abrasive particles. Abrasive particles tend to accumulate near the front edge of the sealing area. The abrasive conglomerates will wear away the seal to result in a secondary wear region. This, in turn, allows more accumulation of the abrasive particles and accelerates the penetration of wear particles into the sealing area.
Ion-Beam Assisted Deposition of Inorganic Coatings for Elastomeric Seal Wear Resistance Improvement:
The present innovations relate to elastomeric seals for use in drill bits on which an inorganic surface modification material has been deposited onto the surface of the preferably elastomeric seals. In a preferred embodiment, the inorganic surface modification material is deposited onto the surface of the seals by means of ion-beam assisted deposition (IBAD), though other methods could be used.
Wear resistance of the seal at the dynamic surface is improved by the addition of the inorganic coating. The harder surface provides support for the soft seal surface and also provides protection against the abrasive particles that tend to accumulate near the front end of the sealing area. The abrasive conglomerates will wear away the seal to produce a secondary wear region. This, in turn, allows more accumulation of the abrasive particles a accelerates the penetration of wear particles into the sealing area. Accordingly, the inorganic coating has a tendency not only to reduce the wear in the primary sealing area but also in the secondary wear area.